The present invention relates to an automatic transmission system with no clutch pedal for use on automobiles such as passenger cars, large-size trucks, and buses, and more particularly to such an automatic transmission system having a throttle actuator, a clutch actuator, a gear shift or selection actuator, a pump mechanism for producing a working pressure for actuating the gear shift actuator, and an electronic control unit for producing control signals to control these actuators in response to detected signals indicative of various parameters such as an engine speed, a brake operation, a throttle position, a shift lever position, a clutch releasing position, and a car speed.
One known automatic transmission system of the type described is disclosed in Japanese Laid-Open Patent Publication No. 58-170955. The disclosed prior automatic transmission system includes an electronic control unit for producing control signals to adjust a throttle valve opening, connect and disconnect a clutch, and change transmission gears in response to detected signals representative of an engine speed, a brake operation, a throttle position, a shift lever position, a clutch releasing position, and a car speed. The automatic transmission system also has a hydraulic gear shift or selection actuator for shifting transmission gears, and a pump mechanism for producing a working pressure for actuating the hydraulic gear shift actuator.
The hydraulic gear selection actuator includes a double-acting cylinder with a piston slidably disposed therein and defining two cylinder chambers in the cylinder. By introducing a hydraulic pressure alternatively in the cylinder chambers, the piston is moved in two directions to provide two piston positions. When the same hydraulic pressure is applied simultaneously to the cylinder chambers, the piston can be set to a central position between the two positions. However, since the piston is moved only under hydraulic pressure, the central position can only be established by controlling the hydraulic pressure while checking the piston position with a detector. The prior gear shift actuator is therefore advantageous in that the detector mechanism is necessary, the piston tends to be displaced out of the central position due to a slight difference between the pressures in the cylinder chambers, and the piston does not respond quickly.
There are three shifting shafts for actuating synchromesh systems for the transmission gears, and the shifting shafts are selectively engaged by a single select lever actuated by the gear selection actuator. The gear selection actuator is controlled by the output signal from the electronic control unit to operate the select lever to shift the gears. Once a gear shift lever is shifted to a desired position, the electronic control unit processes the detected signals indicative of the engine speed, the throttle position, the car speed, and the other parameters for automatically connecting and disconnecting the clutch and shifting the gears, without requiring the driver to depress a clutch pedal. The prior art arrangement is however disadvantageous for the following reasons: Since the single gear select lever is coupled to the single gear selection actuator and selectively engages or selects one of the shifting shafts, operation of the synchromesh systems requires the gear selection actuator first to move the gear select lever in a selecting direction to select a shifting shaft, and then to move the shifting shaft in a shifting direction. This two-step process results in a failure to achieve quick gear changing operation. As a consequence, the driving force from the engine is applied to drive wheels with a delayed response, so that no rapid acceleration and deceleration will be effected.
Another problem with the prior art automatic transmission system is that a high hydraulic pressure (for example about 140 kg/cm.sup.2 in the cylinder chambers) is required to actuate the synchromesh systems. When the shift shaft is kept actuated under such a high pressure, the synchromesh systems are subjected to quick wear on engaging components thereof and hence is poor in durability.